Crystal for X-ray crystal spectrometer

ABSTRACT

A stepped curved crystal for an x-ray crystal spectrometer and the like providing more effective area and hence improved signal to background ratio over conventional curved crystals. A stepped crystal formed of a plurality of crystal strips assembled in a flat configuration and bent to place the steps on a focal circle of radius r, with the crystal atomic planes on circles of radius substantially 2r.

United States Patent Wittry Dec. 16, 1975 CRYSTAL FOR X-RAY CRYSTAL SPECTROMETER Primary Examiner-Craig E. Church 75 Inventor: David B. Wittry, Pasadena, Calif. Agen Kern Wane &

[73] Assignee: University of Southern California,

Los Angeles, Calif.

[57] ABSTRACT [22] Filed: June 28, 1974 A stepped curved crystal for an x-ray crystal spec- [21] PP N04 484,165 tromet er and the like providing more effective area and hence improved signal to background ratio over [52] US. Cl. 250/273; 250/280 conventional curved crystals- A Stepped crystal [51] Int. Cl. G01N 23/20 formed of a plurality of crystal Strips assembled in a [58] Field of Search 250/280, 272, 273, 274 flat Configuration and bent to Place the Steps a focal circle of radius r, with the crystal atomic planes 5 References Cited on circles of radius substantially 2r.

UNlTED STATES PATENTS 9/1969 Okano 250/280 9 Claims, 11 Draw ving Figures US. Patent Dec. 16, 1975 CRYSTAL ROWLAND CIRCLE X- RAY SOURCE HAM CRYSTAL FOR X-RAY CRYSTAL SPECTROMETER This invention relates to crystals for use in x-ray crystal spectrometers and other instruments utilizing a crystal monochrometer for obtaining monochromatic x radiation. The crystals of the present invention are particularly suited for use with x-ray sources of very small dimensions, such as the electron microprobe wherein it is dilficult to achieve a high efficiency without sacrificing the signal to background ratio. It is a specific object of the invention to provide a new and improved crystal having an improved signal to background ratio without adverse effect on the collection efficiency.

X-ray crystal spectrometers are well known and several forms are shown in the following US. Pat. Nos: 2,805,343; 3,070,693; 3,445,653; 3,612,861; 3,772,522; and 3,777,156.

Cylindrically curved. crystals are used in many of the prior art instruments. The prior art Johann crystal is usually made from a strip or plate of crystal having flat surfaces parallel to its atomic planes. The crystal is then bent to a cylindrical configuration with the inner surface having a radius twice the radius of the focal circle of the instrument in which the crystal is to be used. The focal circle is also known as the Rowland circle. In the prior art Johannson crystal, the same flat crystal may be bent to the same curved configuration and then the inner surface is ground to a cylindrical curve having a radius the same as the focal circle. Alternatively, a cylindrically curved plate of uniform thickness may be fabricated by grinding both sides to a radius of 2r. This plate may then be bent to a radius z r to provide crystal planes curved to a radius of twice the focal circle radius and a crystal surface tangent to the focal circle. Side views of these crystals are shown in FIGS. 2, 3 and 3a, respectively, where the thickness is exaggerated and the crystal planes are indicated by the interior parallel lines.

Improvement of the resolution and the signal-tobackground ratio of curved-crystal spectrometers may easily be obtained by using crystals that have a small width of the rocking curve and are elastically bent instead of plastically bent, and by choosing the crystal size and shape so that the angle of incidence of the x-rays on the crystal planes does not vary by more than the width of the rocking curve. However, this procedure, regardless of whether it is employed with Johann or Johannson type crystals results in a serious loss of intensity. This loss of intensity, which may be as much as a factor of 6 in going from a plastically bent LiF crystal to an elastically bent quartz crystal, is a serious disadvantage when the same crystal must also be used for measurement of high signal levels. For such a case, relative precision would be reduced by a factor of 6', requiring an increase in counting time of a factor of 6 to obtain the same precision.

It is an object of the present invention to provide a new and improved curved crystal similar to the Johannson type, while having a higher signal to background ratio and not requiring accurate grinding and polishing of cylindrical surfaces.

By way of summary, the crystal of the present inven tion has a stepped surface configuration, which may be made by building the stepped configuration with sandwiched flat crystals having parallel surfaces and atomic 2 planes. The sandwiched structure is bent to place the steps substantially on the focal circle and place the crystal atomic planes on circles of radius twice that of the focal circle.

US. Pat. No. 3,469,098 shows an x-ray analyzing element with a stepped configuration for use as an approximation to the Johannson crystal when the diffracting medium employed is such that the curvature of the surface cannot be altered by grinding. However the diffractin g element involved in this prior patent is not a true single crystal, being formed by a series of monomolecular layers of soap film deposited sequentially on a base having the desired configuration. Devices of this nature are sometimes referred to as pseudocrystals since they can be used as substitutes for true crystals in certain limited applications. However pseudocrystals are of limited utility and are used in practice only for diffraction of x-rays for which true crystals of sufficiently large lattice parameter cannot be obtained. In addition to being limited to pseudocrystals, the prior patent fails to recognize that the stepped configuration can be more than just an approximation to the Johannson geometry; the stepped configuration can actually provide superior performance over the Johannson geometry when the stepped configuration is fabricated with single crystal materials.

Other objects, advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example.

In the drawing:

FIG. 1 is a diagrammatic representation of an x-ray monochrometer incorporating a curved crystal;

FIG. 2 is a side view of the prior art Johann crystal;

FIGS. 3 and 3a are side views of two forms of the prior art Johannson crystals;

FIG. 4 is a view of the inner surface of the crystals of FIGS. 3 and 3a illustrating the effective area of the crystal;

FIGS. 5 and 5a are side views of the presently preferred embodiment of the crystal of the invention showing intermediate steps in its manufacture;

FIG. 6 is a side view of the finished crystal of FIGS. 5 and 5a;

FIG. 7 is a view of the inner surface of the crystal of FIG. 6 showing the effective area;

FIG. 8 is a view of the inner surface of an alternative embodiment of the crystal of FIG. 7; and

FIG. 9 is a view similar to that of FIG. 6 showing another alternative embodiment of the invention.

FIG. 1 diagrammatically illustrates a conventional x-ray crystal spectrometer having an x-ray source 10, a curved crystal l1 and a detector 12 all positioned on a circle 13 of radius r. The spectrometer is operated in the conventional manner with x-rays directed from the source to the crystal and back to the detector. The crystal 11 may be a Johann crystal as shown in FIG. 2 which is formed of a flat crystal with surfaces parallel to its atomic plane, and curved to a radius of 2r. Altematively the crystal 11 may be a Johannson crystal as shown in FIGS. 3 or 3a.

The effective area of a curved crystal is the area of the inner surface which is effective in diffracting the x-rays of a particular wavelength. The effective area of the crystal 1 1 for the Johann case is shown by Ditsman, Bull. Acad. Sci. USSR, Phys Ser. (Eng. Trans) 24, 390,

3 1960, and is illustrated by the hatching in FIG. 4. The unhatched or noneffective area does not contribute to the signal but decreases the signal-to-noise background ratio by diffracting background radiation of different wavelength than the line radiation.

The preferred embodiment of the crystal of the present invention and the preferred method of making it are illustrated in FIGS. 57. A bulk single crystal of conventional material such as quartz, silicon, mica, pyrolytic graphite, lithium fluoride, or the like, is sliced and polished into thin layers or sheets and cut into strips of the desired size. The strips should have parallel flat surfaces which either are parallel to the atomic planes of the crystal or lie at a definite angle to the atomic planes. Crystal strips 15, 16 are positioned on crystal strip 17, and crystal strips 18, 19 are positioned on the strips l5, 16, respectively, to form a stepped configuration such as is shown in FIG. 5. The crystal strips may be bonded together with conventional adhesives such as an epoxy or wax. A transparent adhesive with a different optical index of refraction from the crystal material is preferred for ease in checking the uniformity of the laminated structure by optical interference methods.

The sandwiched or laminated structure of FIG. 5 with the adhesive layers 20 is then pressed against an adhesive coated back plate 42 having a concave cylindrical surface of radius substantially twice the radius of the focal circle of the instrument in which the crystal is to be used and held in place until the adhesive is cured or hardened. This may be accomplished in the manner illustrated in FIG. 5a with additional dummy strips 43, 44 having elastic properties similar to that of the strips l9, but without an adhesive layer. The backing plate 42 may be supported to avoid deformation and bending pressure may be applied by a pressing die 45 having a facing 46 of an elastic material. The finished crystal is shown in FIG. 6. The dimensions of the individual crystal strips forming the laminated structure are selected such that the steps will lie on a circle having substantially the radius of the focal circle when the crystal is bent to the curve of radius equal to twice that of the focal circle.

By way of example, a crystal for use in a spectrometer having a focal circle of 4 inch radius might have dimensions of 0.5 inch by 1.5 inches. The back strip 17 would be 0.5 inch by 1.5 inches by 0.0039 inch. The next strips l5, 16 would be 0.5 inch by 0.5 inch by 0.0039 inch and the final strips 18, 19 0.5 inch by 0.25 inch by 0.0l 14 inch. Since the strips 18, 19 are too thick to bend elastically for most types of crystals (the thickness should be no greater than 10 of the bending radius), the strips 18, 19 may be made by stacking a metal sheet (for example, aluminum) 0.0075 inch thick and a crystal sheet 0.0039 inch thick. Alternatively, the length of the last strips along the crystal face could be altered so that three layers of the crystal material 0.0039 inch thick could be stacked to produce the desired thickness.

FIG. 7 is a view of the inner surface of the curved crystal of FIG. 6, with the effective area indicated by the hatch lines, illustrating that the crystal of the invention has an increased effective area over that of the conventional crystal of the same dimensions as shown in FIG. 4.

While rectangular strips are used in forming the crystal of FIGS. 5-7, other shapes may be utilized and one alternative is shown in FIG. 8. Crystal strips 22, 23, 24, 25 having V-shapes are mounted on a rectangular crystal strip 26, resulting in an inner surface with steps which can coincide with the boundaries of the effective area for diffraction of x-rays of a particular wavelength ran e.

Tie background reflection may be further reduced in any of the embodiments of the invention by adding nonreflective material to the surface of the crystal at the noneffective areas. In the embodiment illustrated in FIG. 8, triangular shaped pieces 30, 31, 32, 33 of a material which does not scatter or diffract x-rays coherently, such as a polycrystalline sheet or a polymer, are affixed to the inner face as indicated in FIG. 8. Similar pieces may be added to the nonrefiective (i.e., nonhatched) areas of the crystal of FIG. 7.

When using a crystal material having low absorption for x-rays, the configuration of FIG. 9 may be used with thinner strips 15-19 and with absorbing strips 50-53 positioned therebetween. The strips 50-53 are made of a material that absorbs but does not coherently diffract x-rays, for example a metal with small and randomly oriented crystallites (aluminum, copper, tin, lead, etc.) or a polymeric material.

I claim:

1. A crystal for an x-ray crystal spectrometer and the like, comprising:

a cylindrical curved crystal comprising a plurality of separate strips of bulk single crystal material joined in a sandwich form having a plurality of substantially parallel surfaces in a stepped configuration,

with the steps substantially on a focal circle of radius r, and with the crystal atomic planes on circles of radius substantially 2r.

2. A crystal as defined in claim 1 with said steps in descending and ascending order.

3. A crystal as defined in claim 1 including strips of x-ray absorbing material between said crystal strips.

4. A crystal as defined in claim 1 with said steps straight and parallel to each other.

5. A crystal as defined in claim 1 with said steps V shaped, with apices of steps on opposite sides of a central portion directed toward each other.

6. A crystal as defined in claim 1 with x-ray nonreflecting material on the non-effective areas of said surfaces.

7. A method of making a crystal for an x-ray crystal spectrometer and the like, including steps of:

making a stepped sandwich of a plurality of separate strips of bulk single crystal material having parallel flat surfaces with parallel crystal atomic planes; and

bending the sandwich into a cylindrical curve with the steps substantially on a focal circle of radius r, and with the crystal atomic planes and surfaces on circles of radius substantially 2r.

8. The method of claim 7 including the step of bonding the strips with a transparent adhesive having an tal strips. 

1. A crystal for an x-ray crystal spectrometer and the like, comprising: a cylindrical curved crystal comprising a plurality of separate strips of bulk single crystal material joined in a sandwich form having a plurality of substantially parallel surfaces in a stepped configuration, with the steps substantially on a focal circle of radius r, and with the crystal atomic planes on circles of radius substantially 2r.
 2. A crystal as defined in claim 1 with said steps in descending and ascending order.
 3. A crystal as defined in claim 1 including strips of x-ray absorbing material between said crystal strips.
 4. A crystal as defined in claim 1 with said steps straight and parallel to each other.
 5. A crystal as defined in claim 1 with said steps V shaped, with apices of steps on opposite sides of a central portion directed toward each other.
 6. A crystal as defined in claim 1 with x-ray non-reflecting material on the non-effective areas of said surfaces.
 7. A method of making a crystal for an x-ray crystal spectrometer and the like, including steps of: making a stepped sandwich of a plurality of separate strips of bulk single crystal material having parallel flat surfaces with parallel crystal atomic planes; and bending the sandwich into a cylindrical curve with the steps substantially on a focal circle of radius r, and with the crystal atomic planes and surfaces on circles of radius substantially 2r.
 8. The method of claim 7 including the step of bonding the strips with a transparent adhesive having an optical index of refraction different from that of the strips.
 9. The method of claim 7 including the step of placing strips of x-ray absorbing material between the crystal strips. 